Due to the wide application of the wireless communications nowadays, lower-cost and wider-bandwidth wireless communications devices have been a target of research and development in the industry. In order to satisfy the needs in the wireless applications, the industry has been driven to migrate to the 2-6 GHz frequency range. For wireless LAN applications, industrial standards target the carrier frequency to either the 5.2-GHz band, by the IEEE 802.11a standard, or the 2.4-GHz band, by the IEEE 802.11g standard. Dual-band transceivers are thus proposed by the industry to provide more flexible and convenient usages of the wireless LAN systems.
Among the dual-band transceivers as announced, most of them contain two different mixers in one transceiver chip. Under such a design, circuit of the transceiver chip is made even complicated and dimension of the chip would be made twice that of a single-band mixer, since all the circuit elements are duplicated. Among all the elements inductors at the load are the most space-consuming component, since 4 load inductors, 2 for each mixer, are required in the differential architecture of the circuit. This becomes a main obstacle in reducing the cost of dual-band applications.
In order to shrink the chip size, it is possible to merge two mixers into a single mixer. This may reduce number of inductors at load in the differential architecture from 4 to 2. However, the load of the mixer was designed to operate at only one specific frequency. As a result, such dual-band mixer can only be used in a dual-band receiver, not a dual-band transmitter. A load that can be shared by two bands was proposed by using a band pass filter with a notch at output to divide the pass band into two different bands. See S. Lavasani et al.: “Pseudo-concurrent 0.18/spl mu/m dual-band CMOS LNA,” Radio Frequency Integrated Circuits (RFIC) Symposium, pp. 8-10, June 2003. The circuit was simplified because 2 bands used shared elements. Unfortunately, under such a design an extra inductor was added to the load to introduce the notch. As a consequence, it still required 4 inductors at load to form a differential architecture.
It is thus necessary to provide a novel dual-band mixer wherein elements may be shared by two bands and no additional inductor will be needed.
It is also necessary to provide a dual-band mixer wherein working frequency bands may be easily selected.